In the Firefox Add-ons Manager tab { Ctrl + Shft + A } hit the More button in that extensions listing and then scroll-down and see if there is a Homepage listed. If there is that hyperlink should take you the the "Homepage" for that extension.

Other than that I don't know what to suggest. Without the exact name of that add-on or where you are trying to download it from all I was able to come up via a Google search was an old extension for Firefox from 2007 - which wouldn't be compatible any more. https://aws.amazon.com/developertools/Amazon-S3/771

If you will type s3 Amazon in the search tab under addons...you will get the s3 :// . I am unable to add this addon

I learn something every day ..." I have never used that Search feature in the AOM and it looks greatly enhanced since I blocked the "Get Add-ons" menu, which I see has been renamed to "Search", from showing in the AOM. When that feature was first offered in Firefox it would delay the loading time of the entire AOM Tab until that feature was able to connect with the server that feature used.

The first item - Amazon Assistant for Firefox - I see goes here - https://www.amazon.com/gp/BIT/theamazonapp?ie=UTF8&*Version*=1&*entries*=0 - when you open More,,, and click the Homepage link.

Is that what you want to install? Does it install from that page?

I can't get it to download so I can save it to disk and then open it to inspect the internal coding of it. And I don't need it, so I won't install it - too many unexpected surprises in the past with stuff I don't use, but want to help another use with. Plus I don't test the Beta version any longer. And it may be a Beta problem that doesn't affect Firefox 43 Release.

No dear... www.s3fox.net ...try this...if you can install in your Firefox...

OK, I was able to save that extension to disk and open it up. Every file and folder is dated 2011-02-02, That extension isn't "signed".

https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox#w_what-can-i-do-if-firefox-disables-an-installed-unsigned-add-on

Override add-on signing (advanced users): You can override this setting by changing the xpinstall.signatures.required preference to false in the Firefox Configuration Editor (about:config typed in the URL bar). Support is not available for any changes made with the Configuration Editor so please do this at your own risk.

Note: That pref will be gone in Firefox 44 (Release slated for Jan 26. 2016) so user's of currently un-signed extensions should contact the developers of those extensions and prod them into getting their act together & get their extensions signed by Mozilla. Otherwise the users of those extensions will be SOL come the release of Firefox 44. Mozilla has already pushed back the version in which this new security feature was to be enabled from Firefox 40 to 44. I doubt if there will be any further reprieve.